In recent years the search for oil and gas has moved offshore. Early offshore oil wells were drilled from fixed, bottom-founded structures. Subsequently, methods and apparatus were developed for conducting floating drilling operations. Today, most offshore exploration wells are drilled from floating drill ships. Additionally, deep water production wells are likely to be drilled from floating vessels or structures.
In floating drilling operations a marine riser is used to guide the drill string into the well and to provide a path for conducting the drilling fluid back to the vessel. The riser is connected at its lower end to the blowout preventer located at the subsea wellhead and at its upper end to the drilling vessel. Since the drilling vessel is subject to vertical movement due to the action of waves and tides, a vertically extensible slip joint is placed in the upper end of the riser string to accommodate the vessel's vertical motion. As the drilling vessel heaves, the slip joint telescopes to compensate for the vessel movement.
The riser can buckle under the influence of its own weight and the weight of the drilling fluid contained therein if adequate vertical tension is not maintained at its top. Typically, this is provided by using tensioning devices loctated on the drilling vessel to apply axial tension to the upper end of the riser. The tensioning devices are connected to the lower portion of the slip joint. In this manner the vessel is allowed to freely move up and down while maintaining a relatively constant tension in the riser.
Marine risers have been tensioned in various manners including the use of counterweight systems and pneumatic spring systems. The counterweight was the first technique utilized to apply tension to the top of the marine riser. The weight was hung from a wire rope which was reeved up over wire rope sheaves and down to the top of the riser pipe. The tension was equal to the counterweight and therefore was practical only for shallow water drilling where the amount of tension required is low. A second disadvantage of counterweight systems was that large inertial loads were developed when the vessel's movement was large. The pneumatic spring tensioner systems replaced the counterweight systems as deeper and rougher water drilling evolved. The pneumatic spring tensioning devices use a large volume of compressed air to apply nearly constant tension to the top of the riser through wire ropes. See, Harris, L. P., Design for Reliability in Deepwater Floating Drilling Operations, Chapter 14, "Marine Riser Tensioning System", pages 188-194, The Petroleum Publishing Company, Tulsa, Okla., 1979.
Nearly, all floating drilling vessels are now equipped with pneumatic/hydraulic tensioning systems. A large air supply keeps a nearly constant pressure above oil in an air-oil accumulator cylinder. The oil then provides pressure to the face of the piston. As the vessel heaves, the piston moves up and down against a relatively constant force and the tension lines maintain a relatively constant pull on the riser. A series of sheaves are provided on the tensioner and the reeving typically used will give a piston stroke of about 1/4 of the vessel heave.
The tensioner lines are normally run over fixed sheaves supported from the drilling vessel substructure and attached to a tension ring near the top of the outer barrel of the riser slip joint. An even number of tensioners are generally employed and the lines are equally loaded with opposing pairs on opposite sides of the outer barrel. The angles between the tensioner lines and the riser are minimized by locating the turndown sheaves as close to the axis of the riser as possible so that the maximum vertical tension can be applied to the riser.
One disadvantage of present tensioner systems is that the tensioning lines occasionally fail under high tension. Failure is generally attributed to fatigue caused by continuously bending the wire cable back and forth over the sheaves. When the wire cable fails the unrestrained tensioner piston tends to extend rapidly. Since the force behind the piston is generally very high, this unrestrained movement is likely to cause damage to the tensioning device and potentially to the vessel itself. Past efforts to prevent damage from a broken cable event have included the use of flow limiting valves in the tensioner's hydraulic fluid supply line and orifice plates in the exhaust line to limit the final velocity of the piston. Unfortunately, use of these devices also tends to reduce the efficiency of the tensioning system during periods of normal operation.
A second disadvantage of present tensioner systems occurs in the event of an emergency disconnect of the riser. The drilling vessel may move off station due to the action of wind, waves and currents. Alternatively, the automatic positioning system of a dynamically positioned vessel may fail causing the vessel to move laterally. This lateral movement may cause one or more damaging events. For example, the slip joint may contact the vessel's moonpool or may over extend. Also, the riser's lower ball joint may hit its stop. Typically, risers are equipped with a system which allows rapid uncoupling of the riser from the blowout preventer. This uncoupling sharply reduces the tension in the tensioning lines. In such an emergency situation there is not always time to relieve the pressure in the tensioning system. If the riser is disconnected while the tensioning system is still pressurized, the unrestrained riser will be accelerated rapidly upwardly by the tensioning system causing damage to the drilling rig and the vessel. Flow limiting valves and orifice plates partially solve this problem, however, these devices do not completely arrest the riser's upward motion. Also, as noted above, such devices adversely effect the operating efficiency of the tensioning system during normal operation.
Thus, it is apparent that a need exists for a riser tensioner safety system which will prevent damage during a broken cable event or an emergency disconnect of the riser while permitting maximum operating efficiency during periods of normal operation.